As a plot device, this functions very well. It allows a third person to participate in the encounters between two other characters. Note also that this is a subvocal input unit, as well.

Orne stood at an opposite port, studying the jungle horizon... The surgical scars on his neck where the micro-communications equipment had been inserted into his flesh itched maddeningly...

"Let's check that equipment the surgeons put in your neck."

Stetson put a hand to his own throat. His mouth remained closed, but a surf-hissing voice became audible to Orne, radiating from the implanted transceiver: "You read me, Orne? ...touch the mike contact. Keep your mouth closed. Just use your speaking muscles without speaking aloud."

The idea of an implanted phone is used elsewhere (see the audio relay by Robert Heinlein). However, Herbert uses it as both surveillance as well as straightforward communications. Spying with the device sounds too sneaky for Heinlein.